Hydrazinium nitroformate based high performance solid propellants

ABSTRACT

The present invention is directed to a solid propellant for rocket motors, gas generators and comparable devices, comprising a cured composition of hydrazinium nitroformate and an unsaturated hydroxyl terminated hydrocarbon compound.

The present invention is directed to solid propellants for rocketmotors, gas generators and comparable devices, based on a high energeticoxidizer, combined with a binder material.

Solid propellant combinations are prepared by blending solid oxidizerssuch as ammonium perchlorate or hydrazinium nitroformate with a liquidprecursor for the matrix material. By curing of the binder a solidpropellant is obtained, consisting of a polymer matrix and oxidiser inthe form of solid inclusions.

For ammonium perchlorate quite often liquid hydroxyl terminatedpolybutadienes are used as precursor for the matrix material. However,for hydrazinium nitroformate these precursors were not used, as theywere deemed unsuitable for combination with hydrazinium nitroformate(U.S. Pat. No. 3,658,608 and U.S. Pat. No. 3,708,359). It was expectedthat the hydrazinium nitroformate combination with the polybutadienewould be unstable, due to reaction of the hydrazinium nitroformate withthe double C═C bond.

The present invention is based on the surprising discovery that it ispossible to combine hydrazinium nitroformate with hydroxyl terminatedunsaturated hydrocarbon compounds and accordingly the invention isdirected to a stable solid propellant for rocket motors, comprising acured composition of hydrazinium nitroformate and an unsatured hydroxylterminated hydrocarbon compound.

A chemically stable solid propellant, with sufficient shelf life forpractical use can be obtained, provided that hydrazinium nitroformate ofhigh purity is used, which can, among others, be realized byimprovements in the production process like the use of pure startingmaterials, containing substantially less impurities (e.g. chromium,iron, nickel, copper, and oxides of the metals, ammonia, aniline,solvent and the like).

A chemically stable material shows absence of spontaneous ignitionduring storage at room temperature (20° C.) of at least 3 months,although it is preferred to have an absence of spontaneous ignition forat least 6 months, more preferred one year.

A further improvement in the stability of the solid propellant can beobtained by using hydrazinium nitroformate which contains substantiallyno hydrazine or nitroform in unreacted form. This can for example beobtained by changes in the production process, as discussed in WO-A9410104 and a strict control of the addition rate of hydrazine andnitroform during the production of hydrazinium nitroformate, resultingin a purity of the recrystallised hydrazinium nitroformate between 98.8and 100.3, based on H₃O⁺ and a pH-value of a 10 wt. % aqueous solutionof hydrazinium nitroformate of at least 4. Further, the water content ofthe different propellant ingredients, especially the water content ofthe binder components influences the stability and accordingly a watercontent of less than 0.01 wt. % in the binder is preferred. In additionto the aforementioned aspects, stabilisers may be added to furtherimprove the shelf-life.

Further important variables in the production of the solid propellantare the selection of the curing temperature of the matrix material, thechoice of the curing agent and the curing catalysts and inhibitors.

The solid propellant combinations according to the invention havevarious advantages. They possess an increased performance, expressed asan increased specific impulse for rocket applications and as anincreased ramjet specific impulse for gasgenerator applications. Theramjet specific impulse is defined as: I_(sp,r)=(I=φ)I_(sp)−φU₀/g.

In which φ is the weight mixture ratio of air and gas generatorpropellant, I_(sp) is the specific impulse with ambient air as one ofthe propellant ingredients and U₀ is the velocity of the incoming air.

As the energy content of the system is high, it may become possible touse less oxidiser, thereby increasing the overall performance.

Further, it is to be noted that the material is chlorine free, which isan advantage from both corrosion and environmental considerations.

Depending on the actual use various compositions of the solid propellantaccording to the invention are possible. According to a first embodimenta solid propellant can comprise 80 to 90 wt. % of hydraziniumnitroformate, in combination with 10 to 20 wt. % of binder (hydroxylterminated unsaturated hydrocarbon and other standard binder components,such as curatives, plasticisers, crosslinking agents, chain extendersand anti-oxidants). In case a fuel additive, such as aluminium is added,10 to 20% of the hydrazinium nitroformate in the above composition canbe replaced by the additive. These formulations are especially suited asrocket propellants with improved performance.

For the purpose of a gas generator propellant for ramjets or ductedrockets, the following combinations are preferred. 20 to 50 wt. % ofhydrazinium nitroformate, combined with 50 to 80 wt. % of hydroxylterminated unsatured hydrocarbon. As in the above composition it is alsopossible to use an amount of fuel additive for increased performance,such as Al, B, C and B₄C, whereby this fuel additive may be present in10 to 70 wt. %, in combination with 10 to 70 wt. % of the hydrocarbon,keeping the amount of hydrazinium nitroformate identical.

As indicated above, the solid propellant is prepared from a curedcomposition of hydrazinium nitroformate and a hydroxyl terminatedunsatured hydrocarbon. The hydrazinium nitroformate preferably has thecomposition described above, whereby the amount of impurities is kept ata minimum.

The binder or polymeric matrix material is prepared from a hydroxylterminated unsaturated hydrocarbon. In view of the production process ofthe solid propellant this hydrocarbon preferably has a low molecularweight, making it castable, even when containing substantial amounts ofsolids. A suitable molecular weight for the hydrocarbon ranges from 2000to 3500 g/mol. After blending the solid hydrazinium nitroformate withthe liquid hydrocarbon it can be poured in a container and cured.

Curing is preferably carried out by crosslinking the hydroxyl terminatedhydrocarbon, preferably hydroxyl terminated polybutadiene, with apolyisocyanate. Suitable polyisocyanates are isophorone-di-isocyanate,hexamethylene diisocyanate, MDI, TDI, and other polyisocyanates knownfor use in solid propellant formulations, as well as combinations andoligomers thereof. In view of stability requirements it is preferred touse MDI, as this provides the best stability (longest shelf-life). Theamounts of hydrocarbon and polyisocyanate are preferably selected independence of the structural requirements so that the ratio of hydroxylgroups in the hydrocarbon and the isocyanate groups is between 0.7 and1.2. Curing conditions are selected such that an optimal product isobtained by modifying temperature, curing time, catalyst type andcatalyst content. Examples of suitable conditions are curing timesbetween 3 and 14 days, temperatures between 30 and 70° C. and use ofsmall amounts of cure catalysts, such as DBTD (<0.05 wt. %).

In case further fuel additives are included in the propellant these areadded prior to curing.

Generally speaking, also minor proportions, especially up to no morethan 2.5 wt. % of substances such as phthalates, stearates, metal salts,such as those of copper, lead, aluminium and magnesium, said salts beingpreferably chlorine free, such as nitrates, sulfates, phosphates and thelike, carbon black, iron containing species, commonly used stabilisercompounds as applied for gun propellants (e.g. diphenylamine,2-nitrodiphenylamine, p-nitromethylaniline, p-nitroethylaniline andcentralites) and the like are added to the propellant combinationsaccording to the invention. These additives are known to the skilledperson and serve to increase stability, storage characteristics andcombustion characteristics.

The invention is now further elucidated on the basis of the followingexamples.

EXAMPLE 1

Cured samples of HNF/HTPB formulations with different polyisocyanatesand additives have been prepared. Typical examples are shown in table 1,showing the stability of the compositions as a function of time andtemperature.

For all cured samples (unless stated differently): NCO/OH=0.900; curingtime is 5-7 days at 40° C., after which samples are either stored for anadditional week at 40° C., or at 60° C. for 1-2 days; solid load 50 wt%; additives 2 wt % (and 48 wt % HNF), unless stated differently.

Time @ 40° C. Mass loss @ Time @ 60° C. Mass loss @ VST^(a) CompositionAdditives [days] 40° C. [wt %] [days] 60° C. [wt %] [ml/g] HNF — 7/140.09/0.16 2 0.40 1.73 HNF/HTPB^(b) — 7/13 0.23/0.39 2 1.76 — +IPDI^(c) —7/14 0.21/0.70 — — 15.4 +IPDI^(d) — 6/13 0.47/1.05 — — 10.4 +IPDI^(d)DBTD^(d) 6/13 0.48/0.97 — — 16.4 +IPDI — 7/14 0.21/0.78 1 4.21 — +IPDIpNMA 7/13 0.16/0.31 1 2.51 — +IPDI Aerosil 7/13 0.21/0.84 1 4.05 — +DesmN100 — 7/14 0.11/0.16 2 6.70 16.1 +Desm N100 pNMA 7/13 0.17/0.25 1 1.65— +Desm N100 Aerosil 7/13 0.17/0.19 1 1.03 — +Desm W — 7/14 0.06/0.19 24.41 — +TDI — 7/14 0.28/0.74 — — — +Desm VL — 7 0.130 2 1.963 — +Desm VLAl(OH)₃ 7 0.168 2 0.228 — +Desm VL pNMA 7 0.121 2 0.265 — +Desm VL MgSO₄7 0.035 2 0.184 — +Desm VL pNMA + MgSO₄ 7 0.086 2 0.128 — +Desm VLpNMA + MgSO₄ 6 0.086 2 0.260 0.73 +DOA^(e) + pNMA + MgSO₄ 6 0.131 20.470 0.76 Desm VL HNF/urethane^(f) — 12 1.15 2 0.76 1.24 ^(a)Vacuumstability test (VST) conditions: 48 hrs @ 60° C. ^(b)Uncured sample.^(c)Different lots of HTPB and HNF were used; the NCO/OH ratio is 1.200(instead of 0.900); curing time 1 day at 40° C. ^(d)Different lot ofHTPB was used; 0.01 wt % DBTD was added as a cure catalyst. ^(e)DOAcontent: 20 wt % (on binder). ^(f)Sample containing a 50/50 wt % mixtureof HNF and rasped HTPB/IPDI binder (NCO/OH = 0.900).

EXAMPLE 2 HNF/HTPB as a High Performance Propellant Composition

In table 2 the specific impulse of HNF/HTPB and NF/AL/HTPB combinationsare presented. Similar AP based compositions are presented for reasonsof comparison. From table 2, it becomes apparent that HNF/AL/HTPBcompositions possess higher specific impulses compared to AP/AL/HTPBcompositions of similar solid load, whereas the HNF/HTPB composition hasthe additional advantage of low smoke properties due to the abundance ofAl in the composition (at cost of some performance loss).

TABLE 2 Specific impulse(s) HNF/ AP/AL/HTPB AL/HTPB Solid load w %AP/HTPB HNF/HTPB (19% AL) (19% AL) 80 276.6 290.8 314.2 327.3 82 283.1296.9 318.6 330.8 84 289.9 303.4 324.8 334.3 86 296.9 310.2 329.1 338.288 303.6 317.2 331.7 344.4 90 309.0 324.1 332.9 348.8

Table 2. Comparison of the theoretical performance of HNF/HTPBpropellants compared to conventional AP/HTPB propellants (NASA CET 89calculations, vacuum specific impluse, chamber pressure 10 MPa,expansion ratio 100, equilibrium flow conditions).

EXAMPLE 3

HNF/HTPB as a high performance fuel for a ducted rocket gas generatorfor ramjet applications. In Table 3 the ramjet specific impulses of a30% and a 40% solids HNF/HTPB are listed in comparison to 40% solidsAP/HTPB fuel and a GAP for ducted rocket gas generator propellants. Inducted rockets, fuel rich reaction products of a propellant are injectedinto a combustion chamber where it reacts with oxygen from the incomingair.

From Table 3 it becomes apparent that HNF/HTPB compositions possesshigher ramjet specific impulses compared to other compositions which aremomentary under consideration for ramjet fuel applications. In additionto high performances, HNF/HTPB has the additional advantages that it hasa low signature (HCl free exhaust), potentially a high pressureexponent, increasing the gas generator throtteability and possibly loweroxidator loadings compared to AP-based gas generators, resulting inoverall performance gains.

TABLE 3 Ramjet specific impulse (s) AP/HTPB HNF/HTPB HNF/HTPB Oxygen/(40% (40% (30% fuel ratio GAP solids) solids) solids) 2.5 369.1 298.6304.3 289.6 10 743.0 901.9 936.0 1010.0 15 785.6 981.5 1023.4 1121.1 20799.3 1022.1 1070.1 1182.3 30 783.1 1044.8 1100.7 1234.7 40 737.3 1025.71087.2 1236.4

Table 3. Ramjet specific impulse for three different ducted rocket gasgenerator propellants (NASA CET 89 calculations, chamber pressure 1 MPa,exit pressure 0.1 MPa, exit pressure 0.1 MPa, sea level at 2.5 M,equilibrium flow conditions).

1. A solid propellant for rocket motors, gas generators and comparabledevices, comprising a cured composition of solid hydraziniumnitroformate, an unsaturated hydroxyl terminated hydrocarbon compoundbinder and a curing agent, wherein the hydrazinium nitroformate whendissolved in water as a 10 wt. % aqueous solution prior to beingincorporated into the propellant has a pH of at least
 4. 2. Propellantaccording to claim 1, wherein hydroxyl terminated polybutadiene is usedas the unsaturated hydroxy terminated hydrocarbon compound. 3.Propellant according to claim 2, wherein the molecular weight of theuncured hydroxyl terminated polybutadiene is between 2000 and 3500g/mol.
 4. Propellant according to claim 1, wherein the hydraziniumnitroformate is prepared from hydrazine and nitroform in substantiallyequimolar ratios.
 5. Propellant according to claim 4, wherein the molarratio of hydrazine to nitroform ranges from 0.99:1 to 1:0.99. 6.Propellant according to claim 1, wherein the curing agent comprises apolyfunctional isocyanate.
 7. Propellant according to claim 6, whereinthe polyfunctional isocyanate is a polyisocyanate is selected from thegroup consisting of isophoron di-isocyanate, hexamethylenedi-isocyanate, methylene, diphenyl, disocyanate, toluene 2,4-disocyanateand oligomers thereof.
 8. Propellant according to claim 1, wherein astabilizing agent is present in the composition, selected from the groupconsisting of magnesium salts, aluminum salts, diphenylamine,2-nitrodiphenylamine, p-nitromethyl-aniline, p-nitromethylaniline,centralites and combinations thereof.
 9. Propellant according to claim1, wherein the hydrazinium nitroformate has a purity of between 98.8 and100.3, based on H₃O⁺ and pH-value of a 10 wt. % aqueous solution ofhydrazinium nitroformate of at least
 4. 10. The propellant of claim 7,wherein said polyisocyanate is methylenediphenyldisocyanate.
 11. Aprocess for the preparation of a propellant of claim 1 comprising curinga composition comprising hydrazinium nitroformate having a pH of atleast 4 when dissolved in water as a 10 wt % aqueous solution prior tocuring, an unsaturated hydroxy terminated hydrocarbon and curing agentoptionally in the presence of an accelerator for the curing agent. 12.Process of claim 11 wherein said composition is cured in the presence ofan accelerator for the curing agent.
 13. In a rocket motor, theimprovement comprising using as the solid propellant a cured compositionof claim
 1. 14. A propellant of claim 1 wherein the curing agent andhydrocarbon have a water content less than 0.01 wt %.
 15. A solidpropellent for rocket motors, gas generators and comparable devicescomprising a cured composition of containing hydrazinium nitroformate,an unsaturated hydroxyl terminated hydrocarbon compound binder and acuring agent, said propellant being obtained by preparing a mixture ofsolid hydrazinium nitroformate, an unsaturated hydroxyl terminatedhydrocarbon compound binder and a curing agent, followed by curing saidmixture, the solid hydrazinium nitroformate, prior to being incorporatedinto the propellant, having a pH of at least 4 when dissolved in wateras a 10 wt. % aqueous solution.